The invention relates to a dielectric material for influencing electric fields, a method for preparing such material, and stress control devices, and particularly stress control elements, manufactured utilizing that material.
In electrical installations for medium and high voltage, i.e., from about 10 kV, high potential gradients may occur in areas where the radial field has discontinuities due to changes in the field borders at the locations which are under voltage and which are separated by a dielectric. In such areas, overburdening of the dielectric can easily occur due to field amplification and densification which the dielectric cannot withstand. Terminations, angular plugs, connecting plugs or other connecting elements of shielded high voltage cables are considered as typical examples of such locations; there, the electric field which exists between the exposed end of the conductor and the end of the shield concentrates in the areas close to the shield, such that there is a danger in those areas that the breakdown voltage of the cable insulation and/or the adjacent air layers may be exceeded.
That danger is increased by the fact that in practice, the transmission of electrical energy deals almost exclusively with alternating voltages in which the dielectric losses in dielectric materials can cause rather substantial temperature increases. Such increases in turn normally result in an increase of the dielectric losses so that eventually an accumulation of the factors which stress the dielectric may exceed the dielectric load capability. It is known to counteract this phenomena and resultant dangers by arranging, in the endangered areas, materials or shaped bodies of various geometric design made therefrom, which due to particular properties, e.g. a resistivity which decreases with the potential gradient (U.S. Pat. No. 2,666,876) or a high permittivity (dielectric constant) of e.g. more than 20 (U.S. Pat. No. 4,053,702), are capable of influencing the electric field to make it more uniform. Such materials typically consist of an electrically insulating base material, particularly an elastomeric plastic, with particles embedded therein which give the material the desired properties.
In the case of voltage-dependent resistivity, for example, particles of a semi-conductive material, e.g. silicium carbide, or metal particles may be embedded in the material. In the case of materials exhibiting a high dielectric constant, it is typical to insert particles of a material having a high dielectric constant, for example barium titanate or titanium oxide.
Furthermore, it is known to influence the field distribution in endangered areas in a capacitive manner by arranging metallic electrodes correspondingly. For example, alternate layers of conductive and insulating foil may be wrapped about a cable termination (U.S. Pat. No. 2,276,923), or a sleeve-shaped stress control element which includes annular electrodes (U.S. Pat. No. 3,673,305) may be placed around the cable termination. Still further, deflectors are known for influencing the field distribution capacitively, which comprise a conductive outer portion of elastic material, and a highly insulating elastic interior portion which is gap-free, connected thereto (U.S. Pat. Nos. 3,243,756 and 3,344,391; German Pat. No. 1,465,493).
The above-described known possibilities for providing resistive, refractive or capacitive field control have some disadvantages, even when applied in combination. The embedding of particles of a material having a high permittivity (dielectric constant) normally increases the dielectric losses considerably. The heating caused thereby in turn increase the tendency of thermal decomposition or premature aging of the insulating materials and thus can lead to thermal break-throughs. In the capacitive field control, the edges of the electrodes form strong discontinuities so that the edges must be arranged in areas having little stress, and thus the field control device as a whole must be dimensioned correspondingly large. It has also been found in the application of conical deflectors on cable terminations that between the cone and the parts of the cable termination, small cavities are likely to occur which are highly undesirable because of their unfavorable influence on the electrical field intensity along the deflector casing. Furthermore, there is the danger that proper positioning of such cones cannot always be ensured.
Finally, another solution of the above-described problems has become known, which offers considerable advantages over the above-described known countermeasures. According to that solution, a material is used for influencing electrical fields which comprises a dielectric base material and a conductive material which is finely distributed therein and consists of platelet-like particles of an electrically conductive substance, particularly metal, the platelets being oriented substantially parallel with each other so that when measuring the dielectric constant, different values are obtained when the measuring electrode arrangement used is applied parallel to or perpendicular to the platelet planes (U.S. Pat. No. 3,349,164). As a base material, preferably an easily workable, particularly pourable or die-castable material is used, which also may be resilient and/or temperature resistant and/or weather-proof, if desired. Examples of such materials include polyvinyl chloride in hard or softened forms, butadiene-acrylonitrile elastomers, and the like. Conductive particles such as carbon black, and filler substances may be added; however, the particle size thereof is considerably smaller than that of the electrically conductive platelets. The material is used either in the form of a wrapping band containing the material, or as a pastable or pasty suspension.
In the case of cable terminations, protective coatings are typically prepared at the application site by wrapping around the band, or applying multiple coatings of the suspension. The desired parallel orientation of the conductive platelets is effected in the manufacture of the band by calendering or by the brushing process in the case of painting with the suspension. The material has been found to be very useful in practice. It has low losses and is insignificantly influenced by possible air inclusions. The mode of function is basically capacitive, i.e., homogenization of the field is obtained through the orientation of the conductive particles together with the relatively high permittivity of the base material.
In practice, however, the wrapping-around of a band, or the brushing-on of a paste is frequently difficult, and in any case is troublesome. Furthermore, coatings made in this manner are strongly dependent on the skill of the respective worker with regard to their geometry and their electrical properties.
It has also been proposed to manufacture prefabricated elastic shaped bodies from materials in which particles of a platelet-shaped conductive material are embedded in a dielectric base material (U.S. Pat. No. 3,515,798), as it was known in the case of materials having embedded particles of insulating material of high permittivity (U.S. Pat. Nos. 3,823,334 and 4,053,702). In practice, however, such has not been followed because the orientation of the platelets of conductive material, which was desired and was considered indispensable, could not be obtained in a simple manner in the manufacture of such shaped bodies. More particularly, however, it has been found that the range of application of the above-described material with embedded parallel-oriented particles of conductive material was limited toward high operating voltages. Still further, the break-through voltage decreases strongly with increasing content of conductive platelets. On the other hand, however, it appeared desirable to take advantage of the favorable properties of the described known material with embedded particles of conductive material also in electrical installations with high operating voltages.